Explanation:
Why is Phobos so dark?
Phobos, the largest and innermost of two
Martian moons, is the darkest moon in the entire
Solar System.
Its unusual orbit and color indicate that it may be a captured
asteroid composed of a mixture of ice and dark rock.
The above picture was captured recently by the robot spacecraft
Mars Global Surveyor (MGS) orbiting Mars.
Phobos is a heavily
cratered and
barren moon, with
its largest crater located on the far side.
From MGS images like this,
Phobos has been determined to be covered by perhaps a meter of
loose dust.
Phobos orbits so close to
Mars that from some places it would appear to rise and
set twice a day, but from other places
it would not be visible at all.
Phobos' orbit around Mars is
continually decaying -- it will likely break up with pieces
crashing to the Martian surface in about 50 million years.